This invention relates to the casting of metal strip by continuous casting in a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls that are cooled so that metal shells solidify on the moving roll surfaces and are brought together at a nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a transition piece to a metal delivery nozzle located above the nip, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
When casting steel strip in a twin roll caster, the strip leaves the nip at very high temperatures of the order of 1400° C. and can suffer very rapid scaling due to oxidation at such high temperatures in an air atmosphere. Such excessive scaling of the strip may result in significant rolled-in scale.
To deal with the problem of rapid scaling of strip emerging from a twin roll strip caster, the newly formed strip has been maintained within a sealed enclosure, or a succession of such sealed enclosures, in which a controlled atmosphere or atmospheres is maintained in order to inhibit oxidation of the cast strip. The controlled atmosphere can be produced by delivering non-oxidizing gases to the sealed enclosure or successive enclosures. However, uneven scaling across the strip can cause uneven friction between the strip and work rolls, and uneven steering of the strip through the rolling mill and downstream to the coiler.
Disclosed is a method of selectively oxidizing on the cast strip surface or surfaces to desirably oxidize the cast strip surface or surfaces, decreasing the friction coefficient of the cast strip. The decreased friction coefficient and more even friction coefficient across the strip decreases mill loads for a given reduction in strip thickness decreasing production costs, and produces strip with smoother surfaces providing higher strip yield for an intended purpose. Also with a decreased and more even friction coefficient, control of strip steering at rolling mill and pinch roll upstream from the coiler is improved resulting in more even strip coiling, and less risk of deformities such as camber and less risk of excessive telescoping in coils.
Disclosed is a method of improving control of thin strip produced by continuous casting comprising:                a) assembling a continuous casting apparatus having a pair of counter-rotating casting rolls, positioned to provide a nip there between, and at least two enclosures downstream from the nip,        b) introducing molten metal to form a casting pool supported on the casting rolls above the nip and counter-rotating the casting rolls to form thin metal strip downwardly from the nip,        c) guiding the strip through a first enclosure downstream from the nip, and a set of pinch rolls into a second enclosure providing entry to a rolling mill, and        d) directing oxygen-containing gas having a desired amount of oxygen through the inlets into the second enclosure to provide an atmosphere of 0.5 and 15% oxygen with humidity between 3% and 10% in the second enclosure to oxidize at least one surface of the strip to form a desired more even thickness of scale on the surface of the strip providing reduced mill load, smoother strip surfaces and more stable downstream steering control of the strip.        
The atmosphere in the second enclosure may comprise between 3% and 7% oxygen inclusive or between 5% and 10 or 15% oxygen, and the humidity in the second enclosure may be between 3% and 5%. Also, the scale on the strip may have a thickness of between 0.05 and 4.0 microns, or between 0.2 and 2.0 microns.
In some embodiments, the gas inlets may be disposed in the top portion or bottom portion of the second enclosure directing oxygen-containing gas downwardly or upwardly respectively toward the surface of the strip. In other embodiments, the gas inlets may be positioned in the top portion and bottom portion of the second enclosure directing oxygen-containing gas both downwardly and upwardly toward both the upper and lower surfaces of the thin metal strip. In such embodiments, the gas inlets may be a top and/or a bottom header comprising at least one nozzle in the top and/or bottom portion of the second enclosure adapted to direct oxygen-containing gas downwardly and/or upwardly toward the surface of the thin metal strip as desired.
The gas inlets in the second enclosure may be adapted to deliver oxygen-containing gas to the enclosure in an amount sufficient to form between 0.05 and 4.0 microns or between 0.2 and 2.0 microns of scale on at least one surface of the thin metal strip and to provide a positive pressure within the second enclosure inhibiting ingress of atmospheric air.
Also disclosed is an apparatus for continuously casting thin metal strip comprising:                a) a continuous caster having a pair of counter-rotatable casting rolls laterally positioned to form a nip therebetween through which thin metal strip can be downwardly cast and a metal delivery system adapted to deliver molten metal between the casting rolls above the nip,        b) at least one enclosure positioned downstream from the nip adapted to permit movement of the cast strip therethrough and providing entry to a rolling mill, and        c) gas inlets adapted to deliver oxygen-containing gas having a desired amount of oxygen into said enclosure to provide an atmosphere of 0.5% and 15% oxygen with humidity between 3% and 10% in the second enclosure, adapted to oxidize at least one surface of the thin metal strip to form scale on the strip to a desired scale thickness on the strip surface to provide less mill loading, smoother strip surfaces, and more stable steering control of the strip downstream from the enclosure.        
In some embodiments, the gas inlets may be adapted to deliver oxygen-containing gas having a desired amount of oxygen into the enclosure to oxidize at least one surface of the thin metal strip to form scale on the strip to a desired thickness of between 0.05 and 4.0 microns to provide less mill loading, smoother strip surfaces and steering control of the strip downstream from the enclosure. In other embodiments, the gas inlets may be adapted to deliver oxygen-containing gas having a desired amount of oxygen into said enclosure to oxidize at least one surface of the cast strip to form scale on the thin metal strip to a desired thickness of between 0.2 and 2.0 microns, to provide less mill loading, smoother strip surfaces and steering control of the strip downstream from the enclosure. The atmosphere of said enclosure may be controlled to be between 3 and 7% or between 5 and 10 or 15% oxygen, with the humidity in the second enclosure may be between 3% and 5%.
In some embodiments, the enclosure may have gas inlets in the bottom portion or top portion of said enclosure adapted to deliver oxygen containing gas upwardly or downwardly into the enclosure to oxidize at least one surface of the strip to provide less mill loading, smoother strip surface, and more stable steering control of the strip downstream from the enclosure. In other embodiments, the enclosure may have gas inlets in the top portion and bottom portion of the enclosure adapted to deliver oxygen containing gas downwardly and upwardly into the enclosure toward the thin metal strip to oxidize both the upper and lower opposed surfaces of the strip to provide less mill loading, smoother strip surfaces and more stable steering control of the strip downstream from the enclosure.
In some embodiments, the enclosure may have a lower pressure than components upstream from the enclosure and may have a higher pressure than components downstream from the enclosure, inhibiting the flow of gases upstream in the system. Alternatively, or in addition, the enclosure may have a higher pressure than the external ambient atmosphere inhibiting the ingress of gasses from adjacent external atmospheres into the enclosure.
Other details, objects and advantages of the invention will become apparent as the following description of embodiments of the invention proceeds.